1. Field of the Invention
The present invention relates to a semiconductor device including a circuit constituted by thin film transistors (hereinafter referred to as xe2x80x9cTFTsxe2x80x9d). Particularly the present invention relates to the structure of, for example, an electro-optical device typified by a liquid crystal display device or an EL (electroluminescence) display device, a semiconductor circuit, and an electronic instrument (electronic equipment) using the electro-optical device or the semiconductor circuit of the invention.
Incidentally, the semiconductor device in the present specification indicates any devices functioning by using semiconductor characteristics, and all of the electro-optical device, the semiconductor circuit, and the electronic instrument are included in the semiconductor device.
2. Description of the Related Art
Since a thin film transistor (hereinafter referred to as a xe2x80x9cTFTxe2x80x9d) can be formed on a transparent substrate, development to apply it to an active matrix type liquid crystal display (hereinafter referred to as an xe2x80x9cAM-LCDxe2x80x9d) has been actively made. Since a TFT using a crystalline semiconductor film (typically, polysilicon film) can obtain a high mobility, it is possible to realize a high fineness image display by integrating functional circuits on the same substrate.
Basically, the AM-LCD includes a pixel region (also called a pixel matrix circuit) for displaying an image, a gate driving circuit (also called a gate driver circuit) for driving a TFT of each of pixels arranged in the pixel region, and a source driving circuit (also called a source driver circuit) or a data driving circuit (also called a data driver circuit) for transmitting an image signal to each pixel TFT, which are formed on the same substrate. Incidentally, a region where the gate driver circuit and the source driver circuit are formed is called a driver circuit region.
In recent years, there is proposed a system-on-panel in which in addition to the pixel region and the driver circuit region, a signal processing circuit such as a signal dividing circuit or a xcex3-correction circuit is also provided on the same substrate.
However, since performance required by a circuit is different between the pixel region and the driver circuit region, it is difficult to satisfy all circuit specifications through TFTs having the same structure. That is, under the present circumstances, such a TFT structure has not been attained that both of a TFT constituting the driver circuit region including a shift register circuit or the like in which importance is attached to a high speed operation and a TFT constituting the pixel region (hereinafter referred to as a xe2x80x9cpixel TFTxe2x80x9d) in which importance is attached to a high withstand voltage characteristic are satisfied at the same time.
The present applicant filed an application as to such a structure that the thickness of a gate insulating film is made different between a TFT constituting the driver circuit (hereinafter referred to as a xe2x80x9cdriver TFTxe2x80x9d) and the pixel TFT (Japanese Patent Application Laid-open No. Hei 10-056184, corresponding to U.S. patent application Ser. No. 08/862,895 U.S. Pat. No. 6,316,787). Specifically, the gate insulating film of the driver TFT is made thinner than the gate insulating film of the pixel TFT.
Based on the structure disclosed in the foregoing publication, the present invention makes an improvement with respect to a pixel region. Specifically, an object of the present invention is to provide a structure for forming a storage capacitor which can secure large capacity with a small area.
Another object of the invention is to provide an electro-optical device, typified by an AM-LCD, having high reliability by forming each circuit of the electro-optical device with a TFT having a suitable structure according to its function. Still another object of the invention is to raise reliability of a semiconductor device (electronic instrument) including such an electro-optical device as a display portion.
According to a structure of the present invention disclosed in the present specification, a semiconductor device comprises a pixel region including a pixel TFT and a storage capacitor, and a driver circuit region for driving the pixel region, and is characterized in that:
the storage capacitor is formed of a first storage capacitor and a second storage capacitor connected in parallel with each other;
the first storage capacitor is formed of a first capacitance electrode electrically connected to an active layer of the pixel TFT, a first dielectric, and a second capacitance electrode, and the second storage capacitor is formed of the second capacitance electrode, a second dielectric, and a third capacitance electrode; and
the first capacitance electrode and the third capacitance electrode are electrically connected to each other through a pixel electrode.
In the above structure, as the first dielectric or the second dielectric, an insulating film containing silicon can be used, and it is preferable that the thickness of the first dielectric or the second dielectric is set to 5 to 50 nm (preferably 10 to 30 nm).
In the above structure, by using, as the second capacitance electrode, a gate wiring (gate wiring not selected) adjacent to a gate wiring (gate wiring selected) to which a signal is given, it is possible to prevent an opening ratio (ratio of an area which can be used for image display in a transmission type liquid crystal display device) from lowering.
According to another structure of the present invention, a semiconductor device comprises an active layer, a gate wiring provided opposite to the active layer with a first dielectric interposed therebetween, a first interlayer insulating film covering the gate wiring, an opening portion provided in the first interlayer insulating film, a second dielectric covering the opening portion, a light-shielding film provided opposite to the gate wiring with the second dielectric interposed therebetween, a second interlayer insulating film covering the light-shielding film, a source wiring or a drain wiring on the second interlayer insulating film, a third interlayer insulating film covering the source wiring or the drain wiring, and a pixel electrode on the third interlayer insulating film, and is characterized in that
the active layer and the gate wiring form a first storage capacitor with the first dielectric interposed therebetween, the gate wiring and the light-shielding film form a second storage capacitor with the second dielectric interposed therebetween, and in that
the active layer and the light-shielding film are electrically connected through the pixel electrode.
Also in the above structure, as the first dielectric or the second dielectric, it is possible to use an insulating film having a thickness of 5 to 50 nm (preferably 10 to 30 nm) and containing silicon.
A gate insulating film of a pixel TFT is selectively etched and is removed in a region where the storage capacitor is formed. Thereafter, a thin insulating film (first dielectric) having a thickness of 5 to 50 nm (preferably 10 to 30 nm) is newly formed on the surface of the active layer of the region. That is, the thickness of the gate insulating film of the pixel TFT is thicker than the thickness of the first dielectric.
Moreover, the first interlayer insulating film is also selectively etched in the region where the storage capacitor is formed. Then, an insulating film having a thickness of 5 to 50 nm (preferably 10 to 30 nm) and containing silicon is newly formed to serve as the second dielectric.
According to still another structure of the present invention, a method of fabricating a semiconductor device comprises:
a first step of forming an active layer over a substrate;
a second step of forming an insulating film containing silicon on the active layer;
a third step of exposing a part of the active layer by removing a part of the insulating film containing silicon;
a fourth step of forming a first dielectric on the active layer exposed in the third step;
a fifth step of forming a gate wiring and a second capacitance electrode on the insulating film containing silicon and on the first dielectric;
a sixth step of forming a first interlayer insulating film on the gate wiring and the second capacitance electrode;
a seventh step of exposing a part of the second capacitance electrode by removing a part of the first interlayer insulating film;
an eighth step of forming a second dielectric on the second capacitance electrode exposed in the seventh step;
a ninth step of forming a light-shielding film on the first interlayer insulating film and the second dielectric;
a tenth step of forming a second interlayer insulating film on the light-shielding film;
an eleventh step of forming a source wiring or a drain wiring on the second interlayer insulating film;
a twelfth step of forming a third interlayer insulating film on the source wiring or the drain wiring; and
a thirteenth step of forming a pixel electrode, on the third interlayer insulating film, electrically connected with the light-shielding film and the drain wiring.
In the above structure, it is preferable that an insulating film formed by a thermal CVD method (typically, a low pressure thermal CVD method) and containing silicon is used for the first dielectric or the second dielectric. This is because, since the thickness of the first dielectric or the second dielectric is as thin as 5 to 50 nm, it is preferable to use a high quality film formed by the thermal CVD method.